Drilling fluids are special fluids designed exclusively for drilling through the reservoir section of a subterranean formation. The reasons for using a specially designed drilling fluids include, but are not necessarily limited to (1) to drill the reservoir zone successfully, which is often a long, horizontal drain hole, (2) to minimize damage of the near-wellbore region and maximize the production of exposed zones, and (3) to facilitate the necessary well completion. Well completion may include complicated procedures. Typically, drilling fluids may resemble completion fluids. Drilling fluids may be brines containing only selected solids of appropriate particle size ranges (for instance, salt crystals or calcium carbonate) and polymers. Usually, additives needed for filtration control and cuttings carrying are present in a drilling fluid. As noted, drilling fluids may contain filtration control additives to inhibit or prevent loss of the drilling fluid into the permeable formation. Fluid loss involves the undesired leakage of the liquid phase of drilling fluid containing solid particles into the formation matrix. The resulting buildup of solid material or filter cake against the borehole wall may be undesirable, as may be the penetration of the filter cake into the formation. The removal of filter cake, which sometimes must be done by force, may often result in irreparable physical damage to the near-wellbore region of the reservoir. Fluid-loss additives are used to control the process and avoid potential damage of the reservoir, particularly in the near-wellbore region. Specially designed fluids may be used to be placed next to the reservoir and make a seal. This fluid may be different than the drilling fluid and is often referred to as a “sealing or lost circulation pill”,
Subterranean formations having naturally occurring fractures present a problem because the fractures exacerbate undesired leakage of the liquid portion of the drilling fluid into the formation. Thus, lost circulation fluid may be a major challenge when drilling through such naturally fractured carbonate formations.
Acidizing wells is a conventional process for increasing or restoring the permeability of subterranean formations so as to facilitate the flow of oil and gas from the formation into the well. This process involves treating the formation with an acid to dissolve fines and carbonate scale that are plugging or clogging the pores, thereby opening the pores and other flow channels and increasing the permeability of the formation. Continued pumping forces the acid into the formation, where it etches channels or wormholes. These channels provide ways for the formation hydrocarbons to enter the well bore. However, if the acid is too strong it will undesirably corrode casing, tubing and downhole equipment such as gravel pack screens and downhole pumps, and will also undesirably increase the widths of the naturally occurring fractures making the potential for fluid loss even greater.
It would thus be desirable to discover drilling fluid or a sealing pill, a drilling fluid or other fluid which would have relatively low viscosity in the drilling pipe but which would shortly after leaving the drill bit increase in viscosity and inhibit or prevent fluid leak-off into the formation, while minimizing formation damage, even in the presence of naturally-occurring fractures.